1. Field of the Invention
The invention relates to a boom for draglines.
2. Discussion of Prior Art
A walking dragline typically includes a main housing, a boom which extends upwardly and outwardly from the main housing, and a mast assembly which extends upwardly from the main housing for supporting the boom. Support lines extend between the upper end of the mast assembly and the boom for supporting the boom relative to the mast assembly. The upper end of the boom has thereon a sheave mounted for rotation about a horizontal axis. A hoist rope extends from a bucket hoist mechanism and over the sheave to a bucket for causing vertical movement of the bucket. A drag rope extends from a bucket drag mechanism to the bucket for causing horizontal movement of the bucket relative to the boom. The main housing is supported by a tub that sits on the ground when the dragline is engaged in digging operations. The main housing is supported for pivotal movement relative to the tub in a lateral plane about a vertical axis. A pair of walking mechanisms are mounted on opposite sides of the main housing and are operable for moving the main housing over the ground between digging operations. The main housing includes an operator's cab adjacent to and generally beneath the boom. A stairway extends from the inner end to the outer end of the boom.
The boom comprises a welded frame including longitudinally extending upper and lower pairs of chords and lacing welded between the chords. The lower end of the boom includes a pair of spaced feet, and a horizontal foot axis extends between the feet. The boom is pivotally connected to the main housing at the feet and thus is pivotable relative to the main housing in a vertical plane about the foot axis. The boom has a longitudinal axis which extends between the midpoint of the foot axis and the point of the boom.
The distance between the upper and lower chords is referred to as the depth of the boom. The boom must have a depth which provides sufficient buckling strength in the vertical plane for a specified application. Ordinarily, in order to provide a particular vertical buckling strength, the depth must be greatest in the center of the boom.
When raising the bucket near the sheave, it is necessary for the operator to avoid running the bucket into the sheave and to avoid swinging the bucket into the boom. Ordinarily, because of the depth of the boom, the operator cannot see from the operator's cab the spacing between the sheave and the bucket and the spacing between the boom and the bucket when raising the bucket near the sheave. Tightlining can also cause the bucket to strike the boom.